The constantly inquiring narrative voice that informs every page of Mihail Sebastian’s resonant novel For Two Thousand Years, bears a close resemblance to the one that can be heard in the journal he kept from 1935 to 1944, the year before he was run over and killed by an army truck in Bucharest while on his way to give a lecture on Balzac at the university. Sebastian, who was born Iosif Mendel Hechter in Brăila, a port on the Danube, in 1907, was a rising star in Romanian culture when For Two Thousand Years (De două mii de ani) was published in 1934. He was a respected lawyer, a successful dramatist and a literary critic and commentator on the arts. He had friends who would be famous in middle age: Mircea Eliade, the expert on the subtle differences between the world’s religions; EM Cioran, the maverick philosopher who moved to Paris and became one of the great prose stylists in the French language, and Eugen Ionescu, the future absurdist playwright who Gallicised his first name to Eugène and changed the “u” at the end of his second to an “o” once he, too, had established himself as a Parisian.

Sebastian’s mentor, the man he admired above all others, was also called Ionescu. Not related to Eugen, Nae Ionescu was a philosopher with an interest in politics and economics. The younger man was so in awe of him that he was often tongue-tied in his presence. Even though Nae had begun to express antisemitic opinions, Sebastian gave him the typescript of For Two Thousand Years to read and asked him to write a foreword. Ionescu duly obliged with a venomous diatribe in which he chastised the author he had once praised and encouraged for daring to assert that a Jew could belong to any national community: “It is an assimilationist illusion, it is the illusion of so many Jews who sincerely believe that they are Romanian … Remember that you are Jewish! Are you Iosif Hechter, a human being from Brăila on the Danube? No, you are a Jew from Brăila on the Danube.”

After some reflection, Sebastian told his publisher to go ahead and print, word for hateful word, what Ionescu had written. He was soon to regret his rash decision. Leftwing critics, who included self-proclaimed Zionists, accused him of being antisemitic himself, while those of the infinitely larger rightwing persuasion echoed Ionescu’s sentiments. The Jews, they contested, were responsible for all the ills besetting their beloved country – communism, syphilis and homosexuality being among the most prevalent. In 1935, the wilfully misunderstood writer rose to his own defence in the essay “How I Became a Hooligan” (“Cum am devenit huligan”). It must have occurred to him, before that “low, dishonest decade” reached its end, that he had been fighting a battle that was already lost.

It is thanks to his brother Benu, who secreted the unpublished journal in the diplomatic pouch of the Israeli embassy in Bucharest when he emigrated from Romania to Israel in 1961, that Mihail Sebastian is now regarded as one of the foremost chroniclers of the rise of nazism in civilised Europe. The manuscript remained in the Hechter family’s possession until they thought it safe to send a copy to a reputable publishing house in Romania. Journal 1935-1944 finally appeared in 1996, after the secret police had been disbanded. The book received enormous coverage in the media and Sebastian was suddenly famous, in a way that he never really had been in life. This gentle, cultivated and good humoured man, with his passionate love of classical music, was the subject of widespread controversy. A new generation of conservative critics voiced opinions not entirely dissimilar to Nae Ionescu’s, asserting that Sebastian’s problems were entirely Jewish, with the long dormant phrase “Jewish problem” being resurrected in several reviews. The truth is that Sebastian’s “problems” were of the everyday kind before his friends saw fit to remind him that, for all his cleverness, he was essentially an outcast. In one particularly vivid entry in Journal, he records how the actress Marietta Sadova – always happy to take leading parts in his plays – was “choking with antisemitism”.

Mihail Sebastian.

Yet it was the distinguished academic Petru Cretia who spoke for the majority of readers when he observed, in 1997, that Sebastian was not besmirching lofty national values with his “calm, sad and forgiving revelations”. He went on to describe Sebastian as a “fair-minded (often angelic) witness”. That gets it right. Both the novel and the diary are disconcerting because their overall tone is so reasonable, so painstakingly on the side of common sense and simple human decency. Both the anonymous narrator in the one and the beleaguered diarist in the other remain hopeful when hopelessness and numb despair seem like the only options.

For Two Thousand Years has been available in French for a long time, but Philip Ó Ceallaigh’s excellent translation marks its first appearance in English. It opens in 1923, when Romanian Jews were, rather begrudgingly, granted equality with their Gentile compatriots. The new law makes the unnamed teenage narrator aware of his Jewishness, having given the fact little consideration up till then. He does not want to be a “fellow sufferer” or martyr, unlike another pupil Marcel Winder, who almost relishes the beatings he endures. He prefers to be alone and making his own choices. There’s a telling aside in which he notes: “I’d like to be an antisemite for five minutes. To feel an enemy in myself who must be vanquished.” Perhaps that very thought had come to the 16-year-old Iosif Hechter in his early struggles to make sense of the inexplicable.

The character of Ghiţă Blidaru, the brilliant lecturer and theorist who exerts his influence on the impressionable young student, is said to be based on Nae Ionescu. It is not an unflattering portrait of a complicated individual who would go on to accept money from IG Farben for his “pro-Nazi activities”. In the early chapters, he is a vivid and slightly exasperating presence, whose every word is recorded in a notebook by his devoted follower. It is Blidaru who advises him to study architecture, which he does, becoming an apprentice under, and assistant to, Mircea Vieru, known as the “master”, a modernist of genius, as Sebastian persuades the reader to believe. In this unashamed novel of ideas, it is those of Vieru that are the most palatable to the liberal and liberated conscience of the west. The sentimental and condescending notion of the saintliness of the Romanian peasant (still doing the rounds today) holds no appeal to him as it did, astonishingly, to Nae Ionescu, Eliade and Cioran, when they mistook fanaticism for reason.

The tone sustained throughout the novel is one of dismayed affection, as in the scene where he pays homage to Abraham Sulitzer, “my old Ahasverus”, the wandering bookseller who insists that Yiddish will remain a living language. He shows the disbelieving youngster Yiddish translations of Shakespeare, Cervantes, Dostoevsky and Hardy, while his silent wife Roza looks on. Roza is accustomed to this, for when Abraham doesn’t have a visitor or a customer he talks to his books rather than to her. It is such a felicitous literary conceit that it has to be true. Abraham is disapproving of assimilation, as is Sami Winkler, with whom the narrator has conversations that the two men realise can’t end in agreement, but are amicable nonetheless. Sami leaves Nazi Romania for Palestine. The ardent Marxist, ST Haim, an attractive Francophile, is slung into prison for his subversive views. Sebastian gives breathing space to them all, as he does for Maurice Buret, the soi-disant panderer he befriends in Paris, who might have been imagined by Choderlos de Laclos, and Marin Dronţu, the determined womaniser and heavy drinker with whom he forms a cautious relationship. Each one is afforded a voice, an idiosyncratic way of speaking, that makes them instantly – and, in some cases, memorably – recognisable.

The narrator remains hopeful when numb despair seems like the only option

Sebastian is a novelist who listens to what his people wish to tell him, especially when he disapproves of everything they are saying. He understands, as a man of the theatre, that a conversation can be more revealing than mere exposition allows. For Two Thousand Years is crammed with conversations, with arguments, with outright speechifying. His narrative method, which is not linear by any means, reflects exactly the voluble and volatile society that inspires it. “I preserve an old sense of obligation, an inevitable sympathy, for the isolated or beaten individual. The only pain which I understand directly and instinctively, without needing it explained, is the pain of discouragement.”

On hearing of Nae Ionescu’s death in March 1940, at the age of 49, Sebastian wept for the man he had once revered. A life that should have encompassed lasting intellectual distinction had fizzled out in defeat and failure. He was denied the opportunity of apologising, with hindsight, for the sheer banality of his political opinions, though it remains doubtful that he would ever have done so. Eugène Ionesco, who loved and respected Sebastian, hated his namesake for creating a “stupid and reactionary Romania”. Cioran admitted that he had been wrong in his youth, but Eliade could only allow himself to remark that “communism won”. A lesser human being would have skewered these monsters in his writings, but Sebastian is of an altogether higher order. They are not satirised in For Two Thousand Years, nor are they misrepresented in the scrupulously accurate entries in Journal 1935-1944. “I will never cease to be a Jew, of course,” Sebastian has his architect say at the close of the novel. “This is not a position I can resign from. It’s not a matter of pride or shame.”

These are the sentiments that caused offence to the Zionists in 1934, but they could have been expressed by Primo Levi and Giorgio Bassani when Mussolini gave approval to the racial laws that were introduced in 1938. Two years earlier, in benighted Bucharest, Sebastian had his journalist’s travel pass confiscated and his work as a lawyer dwindled away. Although he was never deported, he spent the rest of his short, splendid life as an exile in the city he had once adorned. He had Beethoven, Mozart and Bach, whose music he heard on a radio that was frequently plagued by interference, to comfort him. He had Proust, too, about whom he wrote perceptively, and Balzac, and the company of a few loyal friends. He fought off the luxury of melancholy as a matter of principle, though he sometimes gave into it. “I am ashamed to be sad,” he noted on 31 December 1944. He had much to look forward to – another love affair, perhaps; the chance of being reunited with his brothers; the freedom to write without the fear of censorship. One likes to think there was a lightness in his step and in his heart as he set off on 29 May 1945, to take his place again in a society ready to accept him and welcome him home.

• For Two Thousand Years by Mihail Sebastian, translated by Philip Ó Ceallaigh, is published by Penguin Modern Classics. To order a copy for £7.99 (RRP £9.99) go to bookshop.theguardian.com or call 0330 333 6846. Free UK p&p over £10, online orders only. Phone orders min p&p of £1.99.